LC-oscillators for generating high purity clock or local oscillation (LO) signals are the key building blocks in wire-line and wireless communication systems. And multiple phase clock and LO signals are required in the modern communication systems. In-phase and quadrature-phase (I&Q) signals are required in the zero-IF receivers for modulation or demodulation, and in image-rejection receivers such as Weaver or Hartley architecture. Multiphase LO or clock signals are also required for the phase-array applications and the half-rate clock-and-data recovery (CDR).
In order to receive or transmit the signals from or to channels at different frequencies, as well as to handle process, voltage and temperature variations, the LC-oscillators are required to be frequency tunable in applications. In general, capacitive tuning methods including varactor tuning and switching capacitor array are mostly used to tune the oscillation frequency of the LC-oscillators. However, the capacitive tuning method has its limitations. For example, the tuned capacitors can load the tank seriously and consequently lead to lower operation frequencies and higher power consumptions. The AM-to-PM noise transformations due to the varactor can degrade the phase noise and the stability of the oscillator. Moreover, the capacitive tuning method requires a sufficient range of tuning voltage, which is not available in the deep sub-micron CMOS technologies.
On the other hand, aggressive scaling of CMOS technologies makes it possible to design and integrate voltage-controlled oscillators (VCOs) at millimeter-wave (MMW) frequencies. Compared with those of the radio frequency (RF) VCOs, the spectrum purity of MMW VCOs are much degraded due to the inferior Q factor of the varactors and the serious AM-PM noise transformation caused by the large VCO gain in the order of GHz/V. Moreover, the serious trade-off between varactor's tuning ratio and Q factor as well as the reduced supply voltage in deep sub-micron CMOS technologies make the varactor-tuning method less effective for MMW VCOs.
Here, the Q factor, also known as the quality factor, is a dimensionless parameter that describes how under-damped an electrical oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency. Higher Q factor indicates a lower rate of energy loss relative to the stored energy of the oscillator. In other words, the oscillations die out more slowly. On the other hand, oscillators with high Q factors have low damping so that they oscillate longer. In electrical resonant systems, the Q factor is determined by the resistance, inductance, and capacitance of the circuit.